How much carburation
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Senior User
Joined: Jul 2003
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From: North of Toronto, Ontario
How much carburation
Just thought I'd start a debate here. I just finished readind a thread over on the 460 forum about the recomended size of carburator for that motor. So I thought I'd ask the same question over here.
Weather a 1bbl, 2bbl, 3X2bbl, 4bbl, 2X4bbl or whatever setup anyone else has invented, how many CFM's does the 300 perform best on? What setups does it not perform on? What is good and what is crap?
Be sure to include carb size and type, manifold type, and any spacers, adapters, heat plates, or other things included in your setup.
Posting Guru
Joined: Jun 2002
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From: redding U.S.A.
How much carburation
that has a lot to do with how 'stock' the engine is.
if it has a worked head, or a bigger cam, etc. that'll change which carb will work the best for that particular motor.
if it has a worked head, or a bigger cam, etc. that'll change which carb will work the best for that particular motor.
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Joined: Jun 2002
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From: redding U.S.A.
How much carburation
My 300 is EFI, but i believe most around here with mildly modded or stock engines are running 350cfm, or 390cfm.
Remeber that this engine won't rev at all unless its heavily modified, so you don't need to feed too much at one time.
Remeber that this engine won't rev at all unless its heavily modified, so you don't need to feed too much at one time.
Postmaster
Joined: May 2002
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From: Broken Arrow, OK
How much carburation
How big the engine is has very little to do with how much carb it needs. How much power it makes is the real determining factor. The average engine needs 1.5 cfm per horsepower, so a 400 hp engine would need a 600 cfm carb, a 390 cfm carb could support 260 hp, etc. This will fluctuate a little depending on how efficient the engine is, but if you work out all the math for how much fuel it takes to make x amount of horsepower it comes out very near 1.5 cfm/hp. If you have a 400 hp 460 or a 400 hp 302 they both need the same amount of air and fuel to make the power.
There is also a forumla based on rpm, volumetric efficiency, and displacement that is often used. It is:
(displacment*(rpm/2)*ve)/1728=cfm
Using that formula and a 300 running at 4000 rpm and 85% ve gives 295 cfm. Generally speaking, the two methods give similar numbers if you do the calculations using honest and accurate numbers.
There is also a forumla based on rpm, volumetric efficiency, and displacement that is often used. It is:
(displacment*(rpm/2)*ve)/1728=cfm
Using that formula and a 300 running at 4000 rpm and 85% ve gives 295 cfm. Generally speaking, the two methods give similar numbers if you do the calculations using honest and accurate numbers.
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Senior User
Joined: Jul 2003
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From: North of Toronto, Ontario
How much carburation
That seems to sound about right. Two sizes of carbs that I have heard commonly are the Holly 390 CFM and a 500 CFM (4bbl of course) Going this big is some of the torque in the bottom lost or not? Also how does a spacer plate under the carb effect the performance and responce of the engine?
What size were the 2bbl carbs that came stock on a 5.0 truck?
What size were the 2bbl carbs that came stock on a 5.0 truck?
Elder User
Joined: Mar 2003
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From: Colorado
How much carburation
Sorry, can't help with the stock 2bbl numbers, but since I have the Holley chart out - 300 cu. in. : 280 cfm @ 4,000 rpm, 350 @ 5000, 420 @ 6,000 - based on the formula provided by Silver Streak.These are the numbers (carb size) recommended for STOCK STREET engines by Holley engineers. For HP, increase flow by 10%. Many people have a tendency to overcarb, and the engine usually still runs - though less efficiently from both a fuel AND horsepower/torque point of view.
Holley 390 4bbl., 2" open spacer on top of a 1/2" 4 hole spacer (can't get the 1/2" spacer off without risking manifold damage), Clifford single plane intake. Bored .060 over, ballanced, ported, polished, mated intake, efi exhaust (2-1/2"single). The Hooker aerochamber muffler sounds good (to me) and I noticed an improvement in HP AND torque over the flowmaster dual chamber 40 - as advertised.
Holley 390 4bbl., 2" open spacer on top of a 1/2" 4 hole spacer (can't get the 1/2" spacer off without risking manifold damage), Clifford single plane intake. Bored .060 over, ballanced, ported, polished, mated intake, efi exhaust (2-1/2"single). The Hooker aerochamber muffler sounds good (to me) and I noticed an improvement in HP AND torque over the flowmaster dual chamber 40 - as advertised.
